From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0176EC4828F for ; Fri, 2 Feb 2024 19:21:55 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rVz6G-0007CY-JZ; Fri, 02 Feb 2024 14:21:20 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rVz6F-0007Bx-7e for qemu-devel@nongnu.org; Fri, 02 Feb 2024 14:21:19 -0500 Received: from smtp-out2.suse.de ([195.135.223.131]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1rVz6D-0006uf-97 for qemu-devel@nongnu.org; Fri, 02 Feb 2024 14:21:18 -0500 Received: from imap1.dmz-prg2.suse.org (imap1.dmz-prg2.suse.org [IPv6:2a07:de40:b281:104:10:150:64:97]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 7ACF21F78C; Fri, 2 Feb 2024 19:21:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1706901675; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ID9BqHT7EguY50EL68PYvz9N3Yh63osxYiMsZe2FJ80=; b=BS/doIEEC6uoPcX7xYa2h2LRWh19sIjM4gJktA/uUeT6r2XQ50xj+Gy9Gco4ivhFufU52Q DQQqLI6G2KwD+iiac/fCd1Qgz0HMfIX3QD/OTovZZt6lEtLr4O+ggciKMfXRyr4iBhbria 3nrl0lOIyqlFW99jBkkfEWtTha7ybRo= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1706901675; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ID9BqHT7EguY50EL68PYvz9N3Yh63osxYiMsZe2FJ80=; b=yJRhz7WR//r01d15fWWsUHyXZbGYiRDyQ8KRYS/xTtjWOVkZwHb+ViOIMMXnNwxyxIJZcP h441nN1G+IGEZbBw== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1706901675; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ID9BqHT7EguY50EL68PYvz9N3Yh63osxYiMsZe2FJ80=; b=BS/doIEEC6uoPcX7xYa2h2LRWh19sIjM4gJktA/uUeT6r2XQ50xj+Gy9Gco4ivhFufU52Q DQQqLI6G2KwD+iiac/fCd1Qgz0HMfIX3QD/OTovZZt6lEtLr4O+ggciKMfXRyr4iBhbria 3nrl0lOIyqlFW99jBkkfEWtTha7ybRo= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1706901675; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=ID9BqHT7EguY50EL68PYvz9N3Yh63osxYiMsZe2FJ80=; b=yJRhz7WR//r01d15fWWsUHyXZbGYiRDyQ8KRYS/xTtjWOVkZwHb+ViOIMMXnNwxyxIJZcP h441nN1G+IGEZbBw== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 06130139AB; Fri, 2 Feb 2024 19:21:14 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 9mvDL6pAvWWjZgAAD6G6ig (envelope-from ); Fri, 02 Feb 2024 19:21:14 +0000 From: Fabiano Rosas To: peterx@redhat.com, qemu-devel@nongnu.org Cc: Hao Xiang , Bryan Zhang , peterx@redhat.com, Avihai Horon , Yuan Liu , Prasad Pandit Subject: Re: [PATCH v2 06/23] migration/multifd: Separate SYNC request with normal jobs In-Reply-To: <20240202102857.110210-7-peterx@redhat.com> References: <20240202102857.110210-1-peterx@redhat.com> <20240202102857.110210-7-peterx@redhat.com> Date: Fri, 02 Feb 2024 16:21:11 -0300 Message-ID: <87ttmqhdug.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain Authentication-Results: smtp-out2.suse.de; dkim=pass header.d=suse.de header.s=susede2_rsa header.b="BS/doIEE"; dkim=pass header.d=suse.de header.s=susede2_ed25519 header.b=yJRhz7WR X-Spamd-Result: default: False [-3.31 / 50.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; BAYES_HAM(-3.00)[100.00%]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; MIME_GOOD(-0.10)[text/plain]; RCVD_COUNT_THREE(0.00)[3]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+]; MX_GOOD(-0.01)[]; RCPT_COUNT_SEVEN(0.00)[8]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.de:dkim,suse.de:email]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[] X-Rspamd-Server: rspamd1.dmz-prg2.suse.org X-Rspamd-Queue-Id: 7ACF21F78C Received-SPF: pass client-ip=195.135.223.131; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org peterx@redhat.com writes: > From: Peter Xu > > Multifd provide a threaded model for processing jobs. On sender side, > there can be two kinds of job: (1) a list of pages to send, or (2) a sync > request. > > The sync request is a very special kind of job. It never contains a page > array, but only a multifd packet telling the dest side to synchronize with > sent pages. > > Before this patch, both requests use the pending_job field, no matter what > the request is, it will boost pending_job, while multifd sender thread will > decrement it after it finishes one job. > > However this should be racy, because SYNC is special in that it needs to > set p->flags with MULTIFD_FLAG_SYNC, showing that this is a sync request. > Consider a sequence of operations where: > > - migration thread enqueue a job to send some pages, pending_job++ (0->1) > > - [...before the selected multifd sender thread wakes up...] > > - migration thread enqueue another job to sync, pending_job++ (1->2), > setup p->flags=MULTIFD_FLAG_SYNC > > - multifd sender thread wakes up, found pending_job==2 > - send the 1st packet with MULTIFD_FLAG_SYNC and list of pages > - send the 2nd packet with flags==0 and no pages > > This is not expected, because MULTIFD_FLAG_SYNC should hopefully be done > after all the pages are received. Meanwhile, the 2nd packet will be > completely useless, which contains zero information. > > I didn't verify above, but I think this issue is still benign in that at > least on the recv side we always receive pages before handling > MULTIFD_FLAG_SYNC. However that's not always guaranteed and just tricky. > > One other reason I want to separate it is using p->flags to communicate > between the two threads is also not clearly defined, it's very hard to read > and understand why accessing p->flags is always safe; see the current impl > of multifd_send_thread() where we tried to cache only p->flags. It doesn't > need to be that complicated. > > This patch introduces pending_sync, a separate flag just to show that the > requester needs a sync. Alongside, we remove the tricky caching of > p->flags now because after this patch p->flags should only be used by > multifd sender thread now, which will be crystal clear. So it is always > thread safe to access p->flags. > > With that, we can also safely convert the pending_job into a boolean, > because we don't support >1 pending jobs anyway. > > Always use atomic ops to access both flags to make sure no cache effect. > When at it, drop the initial setting of "pending_job = 0" because it's > always allocated using g_new0(). > > Signed-off-by: Peter Xu Reviewed-by: Fabiano Rosas